Advances in the microelectronics industry have been underpinned by improvements in the quality of the constituent device materials. Consider for example silicon: in the 1970s the dislocation density was of the order of 103 cm−2, whereas today defect free wafers with a diameter of up to and greater than 400 mm are being used in the production of microprocessors. The characterisation and understanding of defects within semiconductor materials is necessary if device performance is to be enhanced. The structural and opto-electronic properties of a material are interrelated and thus neither can be examined in isolation of the other. Of particular importance is the influence of structural defects on the opto-electronic properties of the material, as these are known to affect carrier diffusion lengths, radiative and non-radiative recombination processes.
Photoacoustic spectroscopy (PAS) is a non-invasive photo-calorimetric technique that can probe the non-radiative thermal de-excitation channels of a sample and hence compliments absorption and other spectroscopic analysis methods. Only light absorbed within the sample can generate a photoacoustic response and thus, elastic scattering or transmission of light through the sample does not influence this highly sensitive technique. Photoacoustic spectroscopy can be used to measure amongst others, the absorption spectrum, lifetime of photo-excited species and thermal properties of a sample.
The photoacoustic effect was first reported in 1880 by Alexander Graham Bell in a report to the American Association for the Advancement of Science. After the work of Bell, the photoacoustic effect was largely ignored until the latter half of the 20th century because the technical equipment, such as phase sensitive amplifiers and microphones, necessary to obtain accurate results did not exist. The first theoretical description of the photoacoustic effect in non-gaseous samples was made in the early 1970's and several classical extensions were made to this theory before the first semi-classical description of the photoacoustic effect in semiconductors was published in the early 1980's. Essentially, these theories describe how light absorbed in a sample following non-radiative de-excitation processes gives rise to a heat source in the sample that may be distributed throughout the sample volume or confined to its surface. This heat source generates both temperature and pressure fluctuations within the sample, which in turn induce measurable pressure variations within the gas in contact with the sample.
The basic mechanism behind photoacoustic spectroscopy is as follows. Intensity modulated monochromatic light is shone on a sample. Non-radiative de-excitation processes following light absorption consequently heat the sample. By convective processes, the sample in turn heats up a gas layer in the immediate vicinity of the point of light absorption. The modulated nature of the light induces corresponding pressure fluctuations in the gas due to repetitive heating and cooling of the sample. These pressure fluctuations are detected in the case of indirect photoacoustic spectroscopy by a microphone and are known as the photoacoustic signal. A photoacoustic spectrum may be obtained by determining the photoacoustic signal of the sample as a function of the wavelength and modulation frequency of the incident light.
The Extension of PAS to Semiconductors
It will be appreciated that electron excitations, having a finite lifetime, are generated in the process of light absorption. This absorption of light is accompanied by the generation of electron-hole pairs, which exist for a finite lifetime and move within the sample, before transferring their energy back to the sample in the form of heat.
It is known that a photoacoustic spectrum can be used in an evaluation of the optical absorption coefficient and the bandgap energy of a semiconductor sample. The first theory of the photoacoustic effect in semiconductors was developed in the 1980s by Bandeira et al. Several groups tried to improve their theory, but all quintessentially possessed the same foundations. In their study, Bandeira and co-workers were interested in enhancing the photoacoustic effect from samples with low optical absorption coefficients. To this end, they applied an electric field across the sample perpendicular to the direction the incident photons made with the sample. The subsequent Joule heating enhanced the contribution to the photoacoustic signal from photoexcited carriers in the bulk. The application of this technique was limited to an analysis, non-destructively, of the bandgap of semiconductors, direct or indirect.
This technique is also capable of analysing non-destructively:    1. The energy location of sub-bandgap defect levels, which are the prime cause of non-radiative recombination, and thus are detrimental to optoelectronic device operation.    2. The impact of dislocation generation in strained layer epitaxial systems for modem electronic and opto-electronics materials and devices. In un-strained, defect-free substrate material, one only observes an increase in the photoacoustic signal during the bandgap transition. As an epitaxial layer is grown on the substrate, any induced strain will modify the band-structure, possibly providing alternative non-radiative recombination paths for photoexcited carriers. The presence of such levels would be seen as peaks in the spectrum below the bandgap energy. The energy levels of these defects can be inferred directly from the PAS spectrum.    3. The optical absorption coefficient (β) of the semiconductor, for direct or indirect bandgap materials. Through a knowledge of the normalised photoacoustic spectrum and the thermal diffusion length of the sample, it is possible to determine the optical absorption coefficient of the sample.    4. Elastic and thermoelastic properties of the material under investigation.
Photoacoustic spectrometers for the analysis of gaseous substances are commercially available. However, photoacoustic spectrometers for condensed matter analysis are difficult to obtain and are often unsuitable in their construction to the varied needs of a semiconductor experimentalist. This has been the impetus for the development of in-house systems, which are typically designed for specific experimental conditions and a narrow range of type of materials. The design process for many of these systems has been quite arduous, expensive and very involved.
Photoacoustic spectrometry requires the use of an intensity modulated monochromatic light source to induce the photoacoustic effect in the semiconductor. For this purpose, pulsed and continuous lasers are popular. Due to the inherent wavelength properties of such lasing devices, it will be appreciated that they are only useful over a narrow photonic range, the range of operation of the laser. Zegadi et al. (Rev. Sci. Instrum. 65 (7), July 1994) discuss the use of a non-laser device. They disclose the use of a short arc xenon lamp as a light source in the examination of spectra in the near infrared portion. Although this light source has specific application in the region of interest described in Zegadi, it suffers in that the resolution of the incident light on the sample is not as good as what is achievable using lasers. They nevertheless discuss how they believe the resolution of their apparatus is a high resolution arrangement. It will be appreciated from a review of their disclosure that this reference to high resolution is a reference to for example “high energy resolution” as would be found in a typical energy vs. PA Signal plot.
There is, therefore, a need to provide a photoacoustic system that has an extended wavelength range such that it can be used in the analysis of a wide variety of semiconductor sample types, yet maintains an incident light source of sufficient spatial resolution so as to spatially distinguish the location of any defects detected on the sample.
It is therefore an object of the present invention to provide a spectrometer having a light source whose emission spectra is suitable to effect a radiation of samples of differing semiconductor constituency yet maintains resolution so as to enable a spatial discrimination of the location of detected defects in a sample.
Extension of PAS to Measurement of Dielectric Anisotropy
Dielectric thin films are used in numerous applications in semiconductor device fabrication, e.g. pad oxides, inter-level dielectrics, etc. As device dimensions shrink, a precise knowledge and control is required of the nature of the dielectric constant. Dielectric anisotropy is a state in which the dielectric constant parallel to the one axis is different from the dielectric constant perpendicular to that axis.
To date, measurement of these anisotropies requires a direct measurement of capacitance structures on the material under test. The results are both specific only to that capacitance structure and are obtained invasively.
It is an object of the present invention to provide a method and apparatus for the non-destructive and non-invasive measurement of dielectric anisotropies.